Audio Research Classic 60 power amplifier
A less expensive, less powerful, stereo derivative of the Audio Research Classic 150 monoblock, the 60 features classic Audio Research styling, with the mains and output transformers at the front of the chassis behind a brushed aluminum panel (black is an optional finish), and the tubes, filter caps, and circuit board behind the transformers under a black wire cage. Two silent-running AC fans on the rear of this cage direct cooling air over the two groups of four 6550 output tubes. Inputs are via gold-plated phono jacks, while the outputs are taken from two terminal strips, one per channel, these fitted with non-magnetic brass screws.
The only control is a front-panel on/off switch, this accompanied by two green LEDs and two fuses, one of each for AC mains power, the other of each for the high-voltage regulator for the input and driver stages. Construction is to a high standard, and the parts quality is high.
It is worth going into some detail regarding the design of the Classic 60, as it represents the complexity but also elegance of a modern tube design. (I'm pleased to note that Audio Research continues its custom of including a complete schematic and list of replacement parts in their owners' manuals.)
Looking at the Classic 60's input circuit, the audio signal is low-pass-filtered by an RC network with a -3dB point lying between 200kHz and 2MHz (these frequencies given with source impedances of 10k ohms and 100 ohms, respectively), and is then DC-coupled to the gates of two n-channel JFETs. One seems merely to provide voltage amplification; the other is involved in a more complicated signal path, being followed by a MOSFET source follower which is then AC-coupled to the gate of another n-channel JFET, thus creating an equal-amplitude but opposite-polarity version of the input signal. From then on, the circuitry is fully symmetrical: the polarity-correct (hot) and inverted (cold) versions of the signal feed both a DC-servo amplifier, based on an op-amp chip, and the gates of two arrays of four more JFETs (now MOSFETs), p-channel this time. Each group of four FETs is connected as a parallel source follower, presumably to get enough output current to drive the next stage, which is both halves of a 6CG7/6FQ7 dual-triode tube, again connected in parallel. The HF is also rolled off before the source-follower stage.
One of these tubes is therefore used for each of the hot and cold signals to provide more voltage amplification, the hot and cold signals each then being AC-coupled to the grids of a pair of 6550 output tubes connected in parallel. The screen grids of these tubes are connected to the plate supply, resulting in triode operation. (There is no AC connection from the screens to ground.) As with earlier ARC power amplifiers, while the primary of the output transformer is hung between the plates of the hot and cold pairs of 6550s to give push-pull operation, the output tube cathodes are connected to ground via their respective ends of the transformer secondary. This means that the electrical ground has to be connected to the center-tap connection of the transformer secondary, which is actually the one labeled "4 ohms," not the "0 ohms" connection. This is of no concern in normal use, but when the Classic 60 is used in systems that feature some kind of common-ground switch-box, or with multi-amplifier, servo-controlled loudspeaker systems such as the Infinity RS1b, the ground connection must be made to the 4 ohms terminal if damage is not to result, and the positive speaker terminal connected to the 16 ohm connection.
A consistent feature of ARC power amplifiers has been the cross-coupling of the cathodes of the hot and cold halves of the circuit to give automatic compensation for individual tube characteristics. The Classic 60 is no different, the source outputs of the two parallel FET arrays not only feeding the grids of their own 6CG7/6FQ7 tube but also the cathodes of the other polarity signal's 6CG7/6FQ7, this compensating to a large degree for the difference in individual tube characteristics. Audio Research states that replacement tubes needn't necessarily be matched, though matched pairs will offer "slightly better sonic performance."
When the output tubes are replaced, the new tubes have to have their bias adjusted. As supplied, the 6550s carry a standing current of 65mA, resulting in "enriched" class-AB1 operation. This bias level is adjusted by an automatic circuit and is therefore independent of AC line conditions. A second DC servo circuit, again based on a high-quality op-amp chip, keeps the output tubes in DC balance.
Befitting the symmetrical nature of the circuit, there are two negative feedback paths, one from each end of the output transformer secondary. These connect with the sources of the input FETs, and shunt capacitors across the feedback resistors provide additional high-frequency negative feedback, resulting in extra rolloff above 100kHz or so, reaching (according to my calculations) -3dB at approximately 500kHz.
The power supply is shared between the channels, but is no less thorough in execution. With the exception of the high-voltage rail supplying the output tubes, each of the voltage rails is regulated with an op-amp/series MOSFET combination. The output rail is filtered with four 800µF electrolytic capacitors bypassed with 2µF and 0.01µF plastic-film caps. This much filter capacitance results in a stored energy capacity of approximately 283 joules—a lot! The AC-powered tube heaters are driven from three separate transformer windings, one each for the two driver tubes and each channel's four output tubes.
Footnote 1: Actually, when I was studying electronics for my University of London Bachelor's degree in the late '60s, the transistor was even then too arcane a device for there to be much of the syllabus devoted to it. All the audio circuitry I studied was based on vacuum tubes; even the "digital" circuits I constructed, astable multivibrators and flip-flops, for example, were all-tube.—John Atkinson